In semiconductor chips such as DRAM (dynamic random access memory) chips metal structures (e.g., aluminium lines) are used to electrically connect devices on those chips.
The metal structures for semiconductor chips are formed by first depositing the metal with, e.g., a chemical or physical vapor deposition process, on a substrate. Then a polymeric resist material is placed on the metal layer, which is then patterned through lithographic processes.
After the patterning, some of the resist material is removed so that the remaining resist material forms a mask for the following etching of the metal layer. Typical etching chemistries for plasma etching, e.g., for aluminium layers, employ halogen containing etching materials, like BCl3, BCl3/Cl2 or Br and F-containing gases. In general metal layers are etched by halogen containing plasmas.
After the etching of the metal layer, the remaining residue, containing resist with aluminium oxide, is removed by a dry stripping step usually followed by a wet clean process involving commercial or proprietary solutions. Further the residue can also contain silicon (incorporated from the SiO2 layer below the metal layer) and chlorine (which if of sufficient concentration can initiate corrosion of the amphoteric metal lines). Hence the semiconductor substrate can be further processed.
The processing outlined above is described, e.g., in the book by S. A. Campbell “The Science and Engineering of Microelectronic Fabrication”, Oxford University Press, 2001, which is incorporated herein by reference.
One problem of this known processing is that the residue, containing the polymeric resist with complex aluminium oxide, is difficult to remove in the stripping step without damaging the metal lines, especially aluminium lines. Also, at low dimensional technology nodes (e.g. 90 nm or lower) the use of DUV resist is preferred which can lead to increased sidewall residue thicknesses and reduced strip effectivity.
It is known, e.g., from U.S. Pat. No. 5,545,289 ('289 patent) to strip the polymeric resist and other residues with a plasma containing oxygen and an oxygen activating gas such as CF4 or N2. But as is mentioned in the prior art document, those steps are sometimes ineffective. A passivation step is used to reduce post-etch corrosion problems in the metal lines. The solution described in the '289 patent utilizes a multicycle process of the stripping and passivation steps to improve the removal of the polymeric resist. This makes the process more complex. The '289 patent is incorporated herein by reference.
Furthermore, experience has shown that such known methods are ineffective when the residue has a high metal oxide concentration, e.g., a high aluminium oxide concentration.